1. Field of the Invention
The present invention relates to a technique for cooling a wafer.
2. Description of the Related Art
The recording density of a magnetic recording/reproducing apparatus must be improved, while a magnetic recording/reproducing head that is used to convert a magnetic signal recorded onto an electric signal must also achieve higher performance. Taking, as an example, improving a sensitivity enhancement technique for technical issues about a magnetic recording/reproducing head, a sensor using a tunnel magnetic resistance effect (TMR) with a very high MR ratio is prevalent, and its development has advanced.
For example, as described in a reference; APPLIED PHYSICS LETTERS 86, 092502 (2005), David, et. al., an amorphous film is formed by forming an FeCoB film at room temperature, and an MgO film is formed on this amorphous film upon forming a magnetic tunnel junction (MTJ). An FeCoB amorphous film is formed on this MgO film, and this FeCoB/MgO/FeCoB multilayered structure undergoes a heat treatment at 360° C. for two hours, thus preparing a TMR film which exhibits a 230% magnetic resistance change. This is for the following reason. The amorphous film is formed by forming the FeCoB film at room temperature, and the MgO film is formed on this amorphous FeCoB film to obtain an MgO (001) structure. When the multilayered structure formed by sandwiching the FeCoB film by the MgO films undergoes a heat treatment, FeCo of the FeCoB film crystallizes using the MgO films as a template.
On the other hand, the above reference analyzes a crystal structure by forming an MTJ of an FeCo/MgO/FeCo multilayered structure in place of the FeCoB film as a comparative example. As described in this reference, as a result of this analysis, a CoFe film does not have an amorphous structure by forming it at room temperature, and an MgO film formed on that CoFe film does not have any (001) crystal face.
Also, when a wafer undergoes film formation at a low temperature (for example, a minus region), a possibility of formation of an amorphous film is expected. This is because sputter particles lose their energies by the low-temperature wafer simultaneously with attachment to the wafer, and surface mobility of the particles is suppressed. That is, when an FeCo film is formed by sputtering (or deposition) while maintaining the wafer at a low temperature, an amorphous film is formed to form an MTJ, thus obtaining the same properties as those of the FeCoB/MgO/FeCoB multilayered structure.
As described above, a sputtering apparatus, which holds a wafer at a low temperature, is demanded. In order to realize sputtering in a low-temperature region, low-temperature control of a wafer holding table (wafer stage) is required. Low-temperature control of the wafer stage can be attained by directly attaching a refrigerator to a lower portion of the stage.
On the other hand, sputtering apparatuses adopt a so-called stationary deposition system in which the central axis of a wafer stage matches that of a sputtering cathode (or a sputtering target), and a multi-cathode film formation system in which a plurality of sputtering cathodes are attached to a wafer stage obliquely (or by offsetting the cathode central axes). Especially, the latter multi-cathode film formation system is popularly used since it can attain simultaneous sputtering using a plurality of targets, and can obtain a satisfactory film thickness distribution due to oblique incident film formation.
A case will be examined below wherein film formation is attained by the multi-cathode film formation system while maintaining a wafer at a very low temperature. In the multi-cathode film formation system, since a wafer center and target center are offset, a satisfactory film thickness distribution cannot be obtained unless the wafer is rotated.
Japanese Patent Laid-Open No. 2008-156746 discloses the following technique. That is, in an apparatus which performs sputtering while rotating a wafer, a cooler in which, for example, cooling water cooled to a predetermined temperature is circulated is connected to a wafer stage, and is rotated together with the wafer stage.
However, when a refrigerator having a high refrigerating capability such as a refrigerator using a GM (Gifford-McMahon) cycle is directly connected to a wafer stage so as to set the wafer stage at a very low temperature (for example, 100K or less), it is very difficult to rotate the wafer stage. For example, the GM cycle refrigerator requires a compressor and helium hose, and it is difficult to rotate the wafer stage together with them. A method of mechanically separating the refrigerator and wafer stage and rotating the wafer stage alone may be used. For example, Japanese Patent Laid-Open No. 2003-201565 discloses a deposition film forming apparatus comprising a substrate heating mechanism which includes a heater and is provided with a vacuum chamber, and a substrate holder which is rotatably provided on the substrate heating mechanism via a gap. If the heater is replaced with a cooling mechanism, there may be the following two problems. First, since the substrate holder is positioned between the cooling mechanism and substrate, the substrate can not be cooled unless the substrate holder falls in temperature. Secondly, if the substrate holder sufficiently falls in temperature, the temperature of the substrate can not be lowered to the temperature of the substrate holder due to the thermal resistance. These problems make a cooling efficiency of the substrate very worse.
According to the evaluation previously conducted for the aforementioned structure, the substrate holder made up of copper has a diameter of 200 mm and a thickness of 4 mm and disposed on the cooling mechanism at a space of 0.3 mm. When the cooling mechanism is cooled to 50K in vacuum for two hours, the substrate holder made up of copper kept at room temperature.
Two hours were required for cooling the substrate holder made up of copper to 120K by supplying argon gas with the space of 0.3 mm for cooling. The substrate could not be cooled to be more than 180K due to the thermal resistance between the substrate and holder even if the substrate is put on the cooled substrate holder made up of copper. That is, the aforementioned structure can not efficiently cool a wafer while rotating the wafer.